AbstractPhysiological indices of arousal generally increase when heightened demands are placed on an individual’s cognitive resources. As a consequence, measures such as heart rate are frequently used as one method of assessing changes in workload. In a simulation study with young adult (19-23 yrs.) and late middle age (51-66 yrs.) drivers, heart rate responses were compared during a variety of dual task conditions along with driving and task performance data. During two of the tasks in which younger participants showed significant heart rate acceleration, older drivers, as a group, showed little or no change in heart rate. In this paper we present data on a more detailed analysis of the relationship between heart rate change and performance during one of the dual load conditions, a continuous performance task (CPT). The sample was subdivided into individuals who showed a substantive heart rate acceleration response during the task vs. those who showed little change or heart rate deceleration. Of the 18 younger and 15 older adults in the analysis, 56% of the younger and 27% of the older individuals fell into the heart rate acceleration category. Heart rate response did not correlate with performance on the CPT in the younger subjects. In the older subjects, however, the heart rate acceleration group scored significantly higher on the CPT than those who did not exhibit a pattern of heart rate acceleration. In addition to lower performance on the CPT task, older adults in the non-acceleration group showed a significant drop in driving speed, which is generally interpreted as a compensatory response employed to manage total workload. Overall, the late middle aged drivers who showed a heart rate accelerative response during the CPT task performed better on both the primary and secondary tasks than those that did not. The increase in heart rate in the late middle age drivers in this instance could serve as marker for a variety of important performance mediating variables including relative engagement in the task, availability of resources to invest in the dual tasks, attentional style, or overall flexibility of response. The results suggest the potential value of looking at differences in individual patterns of response in driving behavior studies in addition to overall group behavior. The presence of subtypes of heart rate responders, and the observed performance differences between subtypes in this paradigm, illustrate the importance of these considerations. Other heart rate patterning data from the literature is considered and suggestions for future investigation offered.